The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950° C. is more than one hundred years old.
Conventional aluminium production cells are constructed so that in operation a crust of solidified molten electrolyte forms around the inside of the cell sidewalls. At the cell sidewalls, this crust is extended by a ledge of solidified electrolyte which projects inwards over the top of the molten electrolyte. The solid crust in fact extends between large carbon anodes that dip in the molten electrolyte. To replenish the molten electrolyte with alumina in order to compensate for depletion during electrolysis, this crust is broken periodically at selected locations by means of a crust breaker, fresh alumina being fed through the hole in the crust.
This crust/ledge of solidified electrolyte forms part of the cell's heat dissipation system in view of the need to keep the cell in operation at constant temperature despite changes in operating conditions, as when anodes are replaced, or due to damage/wear to the sidewalls, or due to over-heating or cooling as a result of great fluctuations in the operating conditions. In conventional cells, the crust is used as a means for automatically maintaining a satisfactory thermal balance, because the crust/ledge thickness self-adjusts to compensate for thermic unbalances. If the cell overheats, the crust/ledge dissolves partly thereby reducing the thermic insulation, so that more heat is dissipated through the sidewalls leading to cooling of the cell contents. On the other hand, if the cell cools the crust thickens which increases the thermic insulation, so that less heat is dissipated, leading to heating of the cell contents.
The presence of a crust of solidified electrolyte is considered to be important to achieve satisfactory operation of commercial cells for the production of aluminium on a large scale. In fact, the heat balance and energy consumption are major concerns of cell design, since only about 25% of the energy consumed is used for the production of aluminium. Optimization of the heat balance is needed to keep the proper bath temperature and heat flow to maintain a frozen electrolyte layer (side ledge) with a proper thickness.
In conventional cells, the major heat losses occur at the sidewalls, the current collector bars and the cathode bottom, which account for about 35%, 8% and 7% of the total heat losses respectively, and considerable attention is paid to providing a correct balance of these losses. Further losses of 33% occur via the carbon anodes, 10% via the crust and 7% via the deck on the cell sides. This high loss via the anodes is considered inherent in providing the required thermal gradient through the anodes.
It has been suggested to solve this problem by operating the metal-based anode cells without a crust of solidified electrolyte by using a thermal insulation covering the electrolyte, as for instance disclosed in U.S. Pat. Nos. 5,368,702, 6,402,928, 6,656,340, and publications WO02/070784 and WO03/102274 (all assigned to MOLTECH Invent S.A.) as well as in U.S. Pat. No. 5,415,742 (La Camera/Tomaswick/Ray/Ziegler), and Publications WO02/06565 (D'Astolfo/Hornack), US 2001/0035344 (D'Astolfo/Lazzaro) and US 2001/0037946 (D'Astolfo/Moor). US 2003/0209426 (Slaugenhaupt/Kozarek) discloses a ceramic block for use in a crustless cell as a cover element or cell linings made of a sintered mixture of Al2O3 and at least one of NaF, AlF3, CaF2 and MgF2.
A more conservative approach involves the substitution of emerging carbon anode blocks with anode blocks of similar shape having non-consumable surfaces. U.S. Pat. No. 6,681,106 (D'Astolfo/Bates) discloses massive cermet inert anode blocks protected against thermal shocks and chemical reactants by a soluble solid layer of a mixture of alumina, cryolite and cementitious binder. WO2006/007863 (Ginatta) discloses metal anode blocks for the electrowinning of aluminium that are protected against molten electrolyte and anodically-evolved oxygen by cooling the anodes so as to freeze a skin of electrolyte on the exposed anode surfaces.
Despite previous efforts to develop a cell for operation with the new type of non emerging active anode bodies, there is still a need to provide a covering on the cell's molten electrolyte which is resistant to electrolyte vapours and gases evolved during electrolysis and which has sufficient mechanical resistance.